Brake modifications/calculations

[dave1]

>.... 80MM calipers and
>resultant pads produce more force against the rotor with the same PSI from
>the master cylinder

Using larger calipers or slave cylinders and the same master cylinder:

(More Caliper or Cylinder Force) ~ (More Peddle Travel) * (Same Peddle=
=20
Pressure)

How Much More:

(New Percentage) ( (New Dia / 2 ) * ( New Dia / 2 ) * 314 ) / ( (Old Dia=
=20
/ 2) * (Old Dia / 2) * 3.14 )

For example, when I replaced my 15/16" (.938") rear cylinders with 1 1/8"=20
(1.13") cylinders I get:

( (1.13 / 2 ) * ( 1.13 / 2 ) * 314 ) / ( ( .938 / 2) * ( .938 / 2) *=20
3.14 )
( .565 * .565 * 314) / ( .469 * .469 * 3.14 )
100 / .691 144%

So the force on my rear shoes (given the same peddle pressure) is now 144%=
=20
of what it was before the upgrade and the peddle travel (given the same=20
peddle pressure) is roughly 144% of what it was before the upgrade.

But this assumes that all of the peddle travel is a result of the movement=
=20
of the brake shoes, the compression of the brake shoes and the expansion of=
=20
the drum. In reality much of the peddle travel is a result of compression=20
of the brake fluid (Almost stays the same because the peddle pressure is=20
the same, although there is a bit more fluid to compress) and expansion of=
=20
the brake lines (stays the same). In reality the peddle travel after the=20
upgrade is not 144% but something less. This is consistent with what I=20
experienced after I did the upgrade. In fact, I cant really say that=20
increased peddle travel is all that noticeable. I suspect that much of the=
=20
peddle travel (before and after) is related to compression of the brake=20
fluid and expansion of the brake lines.

Notes:
* is multiply
/ is divide
( ) do the stuff in the parenthesis first
~ means roughly equal to
Works with metric or English.

Dave
Ann Arbor, MI.
73 Sequoia (26'/455/HEI/TBI/160=B0/3.42)

 

[gene]

fluids do not compress

gene

>>.... 80MM calipers and
>>resultant pads produce more force against the rotor with the same PSI from
>>the master cylinder
>
>Using larger calipers or slave cylinders and the same master cylinder:
>
>(More Caliper or Cylinder Force) ~ (More Peddle Travel) * (Same Peddle=
=20
>Pressure)
>
>How Much More:
>
>(New Percentage) ( (New Dia / 2 ) * ( New Dia / 2 ) * 314 ) / ( (Old=
Dia=20
>/ 2) * (Old Dia / 2) * 3.14 )
>
>For example, when I replaced my 15/16" (.938") rear cylinders with 1 1/8"=
=20
>(1.13") cylinders I get:
>
>( (1.13 / 2 ) * ( 1.13 / 2 ) * 314 ) / ( ( .938 / 2) * ( .938 / 2) *=
=20
>3.14 )
>( .565 * .565 * 314) / ( .469 * .469 * 3.14 )
>100 / .691 144%
>
>So the force on my rear shoes (given the same peddle pressure) is now 144%=
=20
>of what it was before the upgrade and the peddle travel (given the same=20
>peddle pressure) is roughly 144% of what it was before the upgrade.
>
>But this assumes that all of the peddle travel is a result of the movement=
=20
>of the brake shoes, the compression of the brake shoes and the expansion of=
=20
>the drum. In reality much of the peddle travel is a result of compression=
=20
>of the brake fluid (Almost stays the same because the peddle pressure is=20
>the same, although there is a bit more fluid to compress) and expansion of=
=20
>the brake lines (stays the same). In reality the peddle travel after the=
=20
>upgrade is not 144% but something less. This is consistent with what I=20
>experienced after I did the upgrade. In fact, I cant really say that=20
>increased peddle travel is all that noticeable. I suspect that much of the=
=20
>peddle travel (before and after) is related to compression of the brake=20
>fluid and expansion of the brake lines.
>
>Notes:
>* is multiply
>/ is divide
>( ) do the stuff in the parenthesis first
>~ means roughly equal to
>Works with metric or English.
>
>
>Dave
>Ann Arbor, MI.
>73 Sequoia (26'/455/HEI/TBI/160=B0/3.42)
>
Genef -- 77PB/ore/ca
GMC MOTORHOME INFORMATION
mr.erf
http://www.california.com/~eagle/

 

[dave1]

>fluids do not compress

Brake fluids are designed to minimize their ability to be compressed but=20
all materials compress to one degree or another. I dont know how much DOT-3=
=20
brake fluid compresses per PSI but I am sure there is a spec somewhere.=20
Even solids are compressible to some degree.

Dave
Ann Arbor, MI.
73 Sequoia (26'/455/HEI/TBI/160=B0/3.42)

 

[steve canyon]

Hi Emery,

Nice post -- I like your set up. Sounds like what I'd like to do myself.
I'm curious how you would judge the pedal pressure now compared to the pedal
pressure with a stock system. Don't know how long it's been since yours was
completely stock so maybe I'm asking for something that's hard to judge.

> My experience with 80 mm calipers on the front and the TSM rear disk brake
> kits showed that the pedal travel was just too much for the stock NAPA or the
> Cinnabar master cylinders to handle. It worked fine when I ha the 80mm front
> calipers and the 1-1/16" rear wheel cylinders. But, after putting on the
> rear discs and calipers, and installing a new Cinnabar master cylinder, my
> brakes failed after about 10,000 miles because the seal on the master
> cylinder piston bottomed in the bore and scuffed the rubber seal causing it
> to leak. By going to a larger master cylinder I got back my pedal travel and
> have much better brakes than I ever had.
>
> Emery Stora
> 77 Kingsley
> Santa Fe, NM